Networking of controllers, sensors, and actuators with the help of a communication system has increased greatly in recent years in the manufacture of motor vehicles. Synergistic effects are of primary concern through a distribution of functions to multiple controllers. In this connection, we speak of distributed systems. Communication among users of the communication system is taking place to an increasing extent over bus systems. Each user has a process computer which is connected via an interface to a communication controller over which the user is connected to the bus system. Communication traffic on the bus system, access mechanisms and receiving mechanisms as well as error processing are regulated according to a protocol.
A controller area network (CAN) protocol known from the related art has established itself in the automotive field. The CAN protocol is an event-controlled protocol, i.e., protocol activities such as transmitting a message are initiated by events having their origin outside the communication system. Unique access to the communication system is obtained by a priority-based bit arbitration. This requires that each communication be assigned a unique priority. The CAN protocol is very flexible because other users and messages can be added with no problem as long as free priorities (message identifiers) are still available.
In cases in which the average utilization of capacity of the bus system is relatively low, there is a very high probability that a user wanting to transmit a message will actually transmit its message immediately or within a very short period of time. Since CAN communication systems are typically designed so that the average utilization of capacity of the bus system is sufficiently low, high-speed access to the bus system is possible in the normal case. The worst case from the standpoint of the communication system, namely when all users want to transmit at all times, however, would mean, strictly speaking, an infinitely long latency in a CAN bus system. This would be true at least for messages having a relatively low priority.
For bus systems a probability may exist that a certain latency will not be exceeded in transmitting messages. FIG. 2 shows a probability distribution of latencies for an event-controlled communication system. The probability distribution has a relatively sharp peak in the vicinity of 0 (the probability for a very short latency is very high), but it extends to infinity (no maximum latency can be guaranteed). The probability distribution shows that an event-oriented communication system is very suitable for the normal case (very high probabilities for short latencies) but is not very suitable for the worst case. This may be further exacerbated if there is a fault in a high-priority user which is permanently transmitting high priority messages and thereby blocking the bus system. Consequently, messages having a lower priority cannot be transmitted. Messages having a low priority then have an infinitely long latency.
An event-oriented bus system is thus very suitable for applications in which the worst case is tolerable but value is attached to a very good performance in the normal case.
The time-triggered protocol for class C (TTP/C) is a relatively new protocol. This is a deterministic protocol, i.e., it is strictly time-controlled, redundancy being fixedly stipulated in the protocol. All communication activities on the bus system are strictly periodic. Protocol activities such as transmitting a message are triggered only by the progression of a (global) time base. Access to the bus system is based on allocation of time ranges in which a user has exclusive transmission rights. This protocol is relatively inflexible because new users can be added only when suitable time ranges have previously been released.
The probability of a user gaining access to the bus system when desired does not depend on prevailing utilization of bus system capacity. FIG. 3 shows a probability distribution of latencies for a deterministic communication system. Latencies depend only on the distance in time from the next transmission time. Since an access request of a user occurs outside the range of influence of the communication system, usually being asynchronous with it, the latency between the access request and the actual transmission of a message is equally distributed over the entire time interval between two transmission times. This is a much broader probability distribution than that with an event-oriented bus system, i.e., the probability of gaining access to the bus system after a very short period of time is much lower. However, this priority distribution is localized, i.e., the probability for any latency is zero. The probabilities are the same in the normal case and in the worst case, and an upper limit for the maximum latency may be given—in contrast with an event-oriented communication system. Deterministic communication systems are thus suitable for applications in which the worst case must be tolerated, even if restrictions must be accepted for the normal case. Therefore, preferred areas for use of deterministically controlled protocols include in particular applications in security-relevant areas (e.g., X-by-wire systems) or applications in which the difference between the normal case and the worst case is not very great.
It is also known from the related art that a time-controlled protocol can be made more flexible by reserving certain time ranges and having an event-controlled transmission of messages take place within the reserved time ranges. Thus, the overall protocol still functions on a time-controlled basis, and messages are transmitted on an event-controlled basis only in certain reserved time ranges. Depending on how access within the reserved time ranges is regulated, handling of the normal case and application-specific individual cases can be improved without losing the basic processability of the worst case (finite maximum latency). A bus system that functions in this way is known in the related art as a byte-flight bus or an SI bus.